1. Field of the Invention
This invention relates to an apparatus for cooling strip in a continuous annealing furnace, and more particularly to an apparatus that cools strip at high cooling rate.
2. Description of the Prior Art
Continuous annealing furnaces, as is well known, are designed to provide heating, short-time soaking, cooling and, when necessary, overaging to steel strip.
For the achievement of the desired strip properties, not only heating (annealing) temperature and soaking time but also the manner of cooling plays an important role. It is believed, for instance, that fast cooling followed by overaging provides good aging and anti-fluting characteristics. The cooling of strip after heating and soaking is accomplished by use of various kinds of cooling mediums. Different cooling rates are employed with different cooling mediums.
With water cooling, considerably high cooling rates, including even those which permit what are known as superhigh-rate cooling, are obtainable. The problem with water cooling is that the shape of strip is apt to get damaged by hardening-induced strains. Contact with water forms oxide films on the surface of strip, the removal of which calls for the provision of an additional device which results in an economical disadvantage.
Cooling by contact with rolls cooled by passing water or other cooling mediums therethrough is a method used for the solution of the problems just described. The problem with this method is as follows: The strip passing through a continuous annealing furnace does not always possess adequate flatness. As such, some portion of the strip may get out of contact with the cooling roll (resulting in uneven cooling), thereby bringing about the deformation of the strip. To avoid this, some strip flattening means should be provided ahead of the point where the cooling roll comes in contact with the strip, at the expense of increased cost.
Another widely used cooling method uses gas jet. Although the cooling rate of this method is lower than that of the water and roll-contact cooling, relatively uniform cooling can be achieved. An example of this type of cooling apparatus was disclosed by the U.S. Pat. No. 3,068,586.
Gas cooling means contained in a vertical continuous annealing furnace comprises several cooling gas chambers provided between rotatable feed rolls at the top and bottom of the furnace over which the strip is passed. Cooling is done by directly shooting forth a stream of cooling gas against the strip from nozzles provided to the cooling gas chamber. To achieve an improvement in the anti-fluting characteristics of the strip, the cooling rate must be increased further. This goal will be achieved by shooting forth a greater amount of gas against the strip. However, the goal will be unattainable if the strip and the nozzle tip are wide apart since the speed of the gas jet is much lower when it reaches the strip than the moment of shooting forth. To achieve the desired goal of cooling under such an unfavorable condition, a very large quantity of gas must be supplied, which is by no means advantageous from the standpoint of capital investment, equipment installation space and running cost. To ensure efficient cooling, the distance between the nozzle tip and the strip should be kept relatively small.
Between the feed rolls at the top and bottom (which are approximately 20 m apart although the distance varies from furnaces to furnaces), the strip travels at a speed of 200 to 1000 m/min. As such, the strip may suffer from the resonance caused by the dislocation eccentricity) of the rolls and the vibration known as fluttering resulting from the shooting force of cooling gas against the strip. When the distance between the nozzle tip and the strip is reduced or the amount of gas supply is increased, the gas is shot forth against the strip surface at a greater speed to cause greater fluttering. When excessive fluttering occurs, the strip may come in contact with a gas ejecting device to damage the device and/or the strip itself. Uneven breadthwise cooling, which might result from such overmuch fluttering, is likely to cause deformation which sometimes and up a serious warp known as cooling buckling.